The invention relates generally to dye transfer printers such as thermal printers, and in particular to the problem of crease formation in successive dye transfer areas of the donor web. Crease formation in the dye transfer area can result in an undesirable line artifact being printed on a dye receiver.
A typical multi-color donor web that is used in a thermal printer is substantially thin and has a repeating series of three different color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent laminating section after the cyan color section.
Each color section of the donor web consists of a dye transfer area that is used for dye transfer printing and pair of longitudinal edge areas alongside the transfer area which are not used for printing. The dye transfer area is about 95% of the web width and the two edge areas are each about 2.5% of the web width.
To make a print, the various color dyes in the dye transfer areas of a single series of yellow, magenta and cyan color sections on a donor web are successively heat-transferred by a print head onto a dye receiver such as paper or transparency sheet or roll. The dye transfer from each transfer area to the dye receiver is done line-by-line widthwise across the transfer area via a bead of selectively heated resistive elements on the print head. The print head makes line contact across the entire width of the color section, but it only heats the dye transfer area, i.e. it does not heat the two edge areas alongside the dye transfer area.
As each color section is used for dye transfer at the print head, the donor web is subjected to a longitudinal tension between a donor supply spool and a donor take-up spool which are rearward and forward of the print head. The longitudinal tension, coupled with the heat from the print head, causes a used color section to be stretched lengthwise at least from the print head to the donor take-up spool. Since the dye transfer area in a used color section has been heated by the print head, but the two edge areas alongside the transfer area have not been heated, the transfer area tends to be stretched more than the edge areas. As a result, the transfer area becomes thinner than the two edge areas and develops a wave-like or ripple distortion widthwise between the edge areas.
After the last line is transferred from a dye transfer area to a dye receiver, and as the used color section is advanced forward from the print head and onto the donor take-up spool, the wave-like or ripple distortion in the transfer area causes one or more creases to form at least in a short trailing or rear end portion of the transfer area that has not been used for dye transfer. The creases tend to spread rearward from the trailing or rear end portion of the used transfer area into a leading or front end portion of an unused transfer area in the next (fresh) color section being advanced to the print head. The creases appear to be created because of the difference in thickness between the used transfer area and the edge areas as they are wound under tension from the print head and onto the donor take-up spool.
When a used color section is wrapped under tension around the donor take-up spool, the edge areas wrap differently on the spool than does the used transfer area because of the difference in thickness between the transfer area and the edge areas. As each additional color section is wrapped around the donor take-up spool, the convolution build-up of the thicker edge areas on the spool becomes significantly greater than the convolution build-up of the thinner transfer areas. This non-uniform winding of the used color section increases the likelihood of one or more creases being created because the convolution build-up of the thicker edge areas on the donor take-up spool adds to the tension and distortion of the used transfer areas.
A problem that can result is that a crease in the leading or front end portion of the unused transfer area of the next (fresh) color section will cause an undesirable line artifact to be printed on a leading or front end portion of the dye receiver when the print head is applied to the crease. The line artifact printed on the receiver is about 0.5 inches in length.
The question presented therefore is how to solve the problem of the creases being created in the unused transfer area of each fresh color section so that no line artifacts are printed on the dye receiver.
According to one aspect of the invention, a method of equalizing web-stretching caused by web tensioning in a dye transfer printer that is for use with a donor web having a dye transfer area and opposite edge areas alongside the dye transfer area, comprises:
heating a dye transfer area to effect a dye transfer from the dye transfer area to a dye receiver; and
heating the edge areas alongside the dye transfer area less than the dye transfer area is heated to effect the dye transfer, but heating the edge areas sufficiently so that they may be stretched by tension substantially the same as the dye transfer area may be stretched, whereby the dye transfer area will not be stretched thinner than the edge areas.
According to another aspect of the invention, a dye transfer printer in which a donor web having a dye transfer area and opposite edge areas alongside the dye transfer area may be stretched by tension, comprises:
means for heating a dye transfer area to effect a dye transfer from the dye transfer area to a dye receiver; and
means for heating the edge areas alongside the dye transfer area less than the dye transfer area is heated to effect the dye transfer, but heating the edge areas sufficiently so that they may be stretched by tension substantially the same as the dye transfer area may be stretched, whereby the dye transfer area will not be stretched thinner than the edge areas.
If the edge areas alongside a dye transfer area being used for dye transfer in the printer are stretched substantially the same as the dye transfer area, the dye transfer area will not be stretched thinner than the edge areas and the likelihood of a wave-like or ripple distortion developing across the dye transfer area will be substantially reduced. As a result, there should not be any creases formed in the next unused transfer area which can cause line artifacts to be printed on a dye receiver in the printer.